1. Field of the Invention
The invention generally relates to a lens, and more particularly, to a fixed-focus lens.
2. Description of Related Art
In recent years, conventional cathode ray tubes (CRTs) are gradually replaced by digital display apparatuses, such as liquid crystal displays (LCDs), digital light processors (DLPs ), and plasma display panels (PDPs), and these digital display apparatuses are widely used in the new generation of display apparatuses. Since digital video signals have advantages of being transmitted and processed without distortion, it is the trend to apply the digital display apparatuses to projection display apparatuses, such as rear projection digital display apparatuses. However, for designers of optical apparatuses, there are a lot of challenges related to fixed-focus lenses used in the projection display apparatus.
For example, in order to achieve good imaging quality, the fixed-focus lens used in the normal projection display apparatus usually has characteristics of little distortion and image aberration, high definition, high contrast ratio, and uniform luminance. Moreover, in addition to good imaging quality, the fixed-focus lens used in the projection display apparatus preferably has a large field of view (FOV), so that the projection display apparatus satisfies the requirement for projecting large frames in small space. Furthermore, in order to enhance light source utilization and luminance uniformity of projected frames, it is preferable for the main light ray near the reduced side in the fixed-focus lens to be substantially parallel to the optical axis. That is, it is preferable for a telecentric angle of the main light ray near the reduced side relative to the optical axis to be as small as possible.
However, for the conventional fixed-focus lens, in order to satisfy the condition, i.e. the main light ray near the reduced side of the conventional fixed-focus lens is substantially parallel to the optical axis, a length of the conventional fixed-focus lens and sizes of lenses used therein are increased. Accordingly, a shorter length of the fixed-focus lens is not provided. Moreover, in order to reduce distortion and image aberration, the number of lenses used therein and the field of view (FOV) thereof are limited, so that the requirement for projecting large frames in small space may not be met. Furthermore, aspheric lenses are generally used in the conventional fixed-focus lens to modify image aberration, so that the characteristics of little distortion and image aberration, high definition, high contrast ratio, and uniform luminance of the frames are provided. However, the aspheric lenses are hard to be manufactured and have high cost, so that the cost of the conventional fixed-focus lens is hard to be reduced.